In general, dot matrix printers can be separated into two categories--dot matrix line printers and dot matrix serial printers. Both categories of printers create images (characters or designs) by selectively printing a series of dots in an X-Y matrix. Dot matrix serial printers include a print head that is moved horizontally back and forth across a sheet of paper, either continuously or in steps. The print head includes a vertical column of dot printing elements. As each column position of a character position is reached during printing, the required number of dot-printing elements are actuated to form dots. A series of thusly created vertical dot columns forms the desired character. Contrariwise, dot matrix line printers include a dot-printing mechanism for creating horizontal lines of dots substantially simultaneously as the paper is stepped through the printer. A series of horizontal lines of dots creates an image, i.e., a row of characters or a design. While the present invention may find use in other areas, because it was designed for use in a dot matrix line printer, it is described in connection with such a printer.
In the past, various types of actuating mechanisms for use in dot matrix line printers have been proposed and implemented. U.S. patent application Ser. No. 512,470, filed July 11, 1983, entitled "Single-Piece Hammer Module," and assigned to the assignee of the present application, describes an actuating mechanism for a dot matrix line printer. The actuating mechanism is incorporated in hammer modules that include a plurality of cantilevered print hammer arms formed of a resilient ferromagnetic material. Mounted on the end of each print hammer arm is an anvil (e.g., a ball) that prints a dot when the associated hammer arm is actuated. Each hammer actuation mechanism comprises a permanent magnet, a post and plates that create a magnetic path between the permanent magnet and the post plus a release coil mounted on the post. In the absence of current through the release coil, the print hammer arm is attracted to the post by the magnetic field produced by the permanent magnet. The attraction stresses the hammer arm. The thusly cocked hammer arm is released by energizing the release coil such that the coil produces a magnetic field that counteracts the magnetic post attraction field created by the permanent magnet. When released, the stored energy resulting from stressing a resilient hammer arm causes the hammer to impact the anvil against a ribbon and create a dot on a print receiving medium.
While dot-printing mechanisms of the type generally described above have a number of advantages over previously developed dot-printing mechanisms for use in dot matrix line printers and, thus, form a significant step forward in this art, it has been found that such dot-printing mechanisms can be improved. In this regard, as discussed above, the hammers of the abovedescribed dot-printing mechanisms are released when a suitable current passes through a release coil. While a hammer is released when current of suitable magnitude and direction passes through a release coil, the magnetic field produced by the current flow does not fully counteract the magnetic field produced by the permanent magnet. Rather, the magnetic field of the permanent magnet is only counteracted to the degree necessary for the stress force stored in the hammer to overcome the permanent magnet attraction force. As a result, after release, the permanent magnet produces a magnetic field that creates a "drag" force on the released hammer. This drag force reduces the impact force applied by the hammer to the anvil. One obvious way of reducing the amount of permanent magnet drag force is by increasing the magnitude of the current applied to the release coil. In many printers this approach is unacceptable because increasing release coil current increases the amount of heat generated by the coil, which may result in the premature destruction of the coil. While the heat problem, to some extent, can be compensated by adding or increasing the capacity of an existing cooling mechanism, this approach increases the cost and complexity of the overall printer.
This invention is directed to increasing hammer force for a given amount of current without increasing the magnitude of the current applied to the release coil. Conversely, for a given level of hammer force, this invention is directed to reducing the magnitude of the current applied to the release coil.